(a) Explain with a neat sketch the essential features of English bond in brick masonry construction. (10 marks) (b) State the…

(a) English bond in brick masonry (10 marks)

English bond is the strongest and most common bond for load-bearing brickwork. It is built of alternate courses of stretchers (bricks laid lengthwise along the face) and headers (bricks laid with their ends showing on the face).

Its essential features are:

• Alternate courses are stretcher courses and header courses, so the face shows stretchers in one course and headers in the next.
• The quoin (corner or stop-end) brick of a header course is a header, and a queen closer, a brick cut lengthwise into half its width, is placed next to it. The queen closer moves the following headers a quarter brick across, so each header lies centrally over the joint of the stretchers below.
• Each course laps the one below by a quarter brick, and the vertical joints of alternate courses are never in a line, so no continuous vertical joint runs through the wall.
• In a one-brick wall the stretcher course shows stretchers on both faces and the header course shows headers across the full thickness, which ties the two faces together.
• Bricks are laid frog up with 10 mm joints, and all vertical joints are filled with mortar.

(b) Choice of cement and reasons (10 marks)

(i) Portland Pozzolana Cement. It is the best choice for mass concrete, hydraulic and marine works, sewers and foundations in aggressive soil, and for plastering and masonry mortars. The pozzolana in it reacts with the free lime released by hydration, giving less lime to leach, lower permeability and heat, and better resistance to sulphates and chlorides. It gains strength more slowly early on, so it needs longer curing.

(ii) Rapid Hardening Cement. It is suited to work where early strength is needed: precast units, early removal of formwork, road repairs, and concreting in cold weather. It has more tricalcium silicate (C₃S) and a finer grind, so it achieves at 3 days the strength that ordinary cement reaches in 7 days. It develops more heat, so it is not for mass concrete.

(iii) Quick Setting Cement. It is used for concreting under water or in running water, for grouting, and for sealing leaks and water-bearing strata, where the concrete must set before it is washed out. The gypsum is reduced and aluminium sulphate or alkali is added, so that the initial set is about 5 minutes and the final set within 30 minutes. The concrete has to be mixed, placed and compacted quickly.

(iv) Low Heat Portland Cement. It is chosen for mass concrete such as dams, thick raft foundations and large piers. It has less C₃S and C₃A and more dicalcium silicate (C₂S), so heat is released slowly and the temperature rise and thermal cracking are limited, though early strength is low.

(v) Air-entraining Cement. It is used for concrete exposed to freezing and thawing and to de-icing salts, such as pavements, canal linings, and structures in cold regions. It contains an air-entraining agent that produces millions of tiny, well-spaced air bubbles (about 3–6% by volume), which give room for freezing water to expand, improve workability and reduce bleeding and segregation. Strength falls by about 5% for each 1% of air.

(c) Diminishing balance depreciation (10 marks)

In the diminishing balance (constant percentage) method the value falls by a fixed fraction r of the value at the start of each year. So the scrap value after n years is S = P(1 − r)ⁿ.

(c)(i) Annual rate. With P = ₹80,000, S = ₹20,000 and n = 10:

(1 − r)¹⁰ = 20,000/80,000 = 0.25

1 − r = 0.25^(1/10) = 0.8706

r = 0.1294, so the value falls by 12.94% each year.

(c)(ii) Depreciation fund after 2 years. Value after 2 years = 80,000 × 0.8706² = 80,000 × 0.7579 = ₹60,629. Check by years: depreciation in year 1 = 80,000 × 0.1294 = ₹10,356, and in year 2 = 69,644 × 0.1294 = ₹9,015.

Depreciation fund after 2 years = 80,000 − 60,629 = ₹19,371 (about ₹19,370).

(d) Marshalling yards (10 marks)

Purpose. A marshalling yard is a railway yard where incoming wagons are received, sorted by destination and made up into new trains. Its functions are to classify and reorder wagons, to detach and attach wagons and locomotives, to store wagons awaiting despatch, to inspect and repair wagons, and to hold a section of the network free from shunting so that through trains keep moving. It improves the use of wagons and the speed of goods movement.

Types. There are three types.

Flat (level) yard. The wagons are pushed by a shunting locomotive over level tracks, and each wagon or cut is moved to its siding in turn. It is simple and cheap to build, but slow and labour-intensive, with the locomotive busy on every movement. It suits small yards with little traffic.

Gravity yard. The yard is laid on a falling gradient in the direction of shunting, so the wagons roll down under gravity to the sorting sidings and are controlled by brakes or retarders. It needs less locomotive shunting than a flat yard, and it is suited to moderate traffic where a suitable natural or built gradient can be had.

Hump yard. A locomotive pushes the rake to the top of an artificial hump, where the wagons are uncoupled one by one. Each wagon then rolls down a steep gradient and along a series of switches into the correct sorting siding, with retarders controlling speed. The yard is the fastest and gives a high capacity and fewer locomotive movements, but has a high cost, and needs trained staff and signalling. It suits large yards with heavy traffic.

(e) Overlay design by the Benkelman Beam Deflection method (10 marks)

Key steps (IRC:81).

• Traffic. Estimate the design traffic in cumulative million standard axles (8.16 t) over the design life (at least 10 years for major roads), from N = 365A[(1 + r)ⁿ − 1]/r × F.
• Survey. Do a pavement condition survey (cracking, rutting) and divide the road into sections of uniform condition, at least about 1 km long each.
• Deflection tests. Load a rear axle to 8170 kg on dual tyres at 5.6 kg/cm² pressure. Take at least 10 points per section, not more than 50 m apart, along the outer wheel path, and place the beam tip between the dual wheels. Read the initial dial reading, the intermediate reading after the truck moves forward 2.7 m, and the final reading after 9 m.
• Corrections (below), then the characteristic deflection, D_c = mean + 2σ for national and state highways, or mean + σ for other roads.
• Overlay thickness. Read from the design chart of characteristic deflection against design traffic, the thickness in bituminous macadam (BM), and convert to other materials. The minimum overlay is 50 mm BM plus a wearing course.

Corrections, and why, when and how.

• Leg correction (why: the front legs of the beam sit in the deflection bowl and move, so the reading understates the true deflection). The apparent deflection is X_a = 2 × (initial − final reading). If the differences from the intermediate and final readings differ by more than 0.025 mm, the true deflection is X_t = X_a + 2.91Y, where Y is twice the difference between the final and intermediate readings. The reported rebound deflection is 2X_t.
• Temperature correction (why: a hot bituminous layer is softer, and deflects more). It applies when the bituminous layer is at least 40 mm thick. Correct to the standard 35 °C by 0.01 mm for each °C: add for a lower pavement temperature and subtract for a higher one.
• Seasonal correction (why: the subgrade is weakest after the monsoon, so tests in the dry season understate the deflection). Multiply the deflection corrected for temperature by a factor from IRC:81 charts, that depends on the type of subgrade soil (sandy or clayey, its plasticity), the annual rainfall, and the moisture content at the time of testing.

The corrected deflections for each section give the mean and standard deviation, then the characteristic deflection, and hence the overlay thickness. If a point differs from the mean by more than one third of the mean, extra points are tested 25 m either side.